Metallic tools for chip-forming machining can be exposed to vibrations induced by either forces or regenerative oscillations (i.e., feed-back vibrations); see “Metal Cutting Theory and Practice” by Stephenson and Agapiou; publisher Marcel Dekker Inc.; ISBN: 0-8247-9579-2. According to this document, vibrations induced by forces are generated by transient cutting forces, whereas regenerative vibrations (also called chatter or oscillations) occur because the dynamic cutting process forms a closed loop. The present invention relates to the damping of only regenerative oscillations (also called self-induced vibrations or feedback vibrations).
Regenerative vibration is so named because of the closed-loop nature of the dynamic cutting process. Each tool pass leaves disturbances in the form of undulations on the workpiece due to the vibrations of the tool and workpiece, and those disturbances produce mechanical feedback vibrations in subsequent passes of the tool. Thus, regenerative vibration can be described as a small transient wave that is machined into the workpiece. That small wave will become the driving force, causing the system to vibrate increasingly again in subsequent passes, i.e., the vibrations from one pass are amplified by those of the next pass. Unstable conditions can cause a small wavelet to develop and increase around the circumference, giving non-acceptable machining results.
Damping of vibration in tools for chip removing machining has previously been achieved by pure mechanical damping, i.e., the tool shaft being formed with a cavity in which is disposed a counter-oscillating mass of, for instance, heavy metal. The weight and location of the mass is tuned in order to provide damping of oscillations within a certain range of frequencies. The cavity is then filled with a viscous liquid, e.g. oil, and is plugged. However, this technique works passably only in those cases where the overhang of the shaft from a fastening device is approx. 4-10 times longer than the diameter thereof. In addition to this limitation, the pure mechanical damping has an obvious disadvantage in that the range of frequencies within which the damping acts, is very limited. An additional inconvenience consists of the strength-wise weakening of the shaft resulting from the presence of that cavity.
In entirely other areas of technology, the development of more efficient, adaptive damping techniques based on the utilization of, among other things, piezo elements has been started. A piezo element consists of a material, most often of a ceramic type, which when compressed or elongated in a certain direction (the direction of polarization), generates an electric field in the same direction. The piezo element usually has the shape of a rectangular plate having a direction of polarization, which is parallel to the major axis of the plate. By connecting the piezo element to an electric circuit, including a control module, and compressing or elongating the piezo element in the direction of polarization, an electric current will be generated and flow in the circuit. Electric resistive components included in the control module will generate heat according to known physics. In doing so, vibration energy is converted to thermal energy, whereby a passively damping, but not damping effect on the vibrations is obtained. Moreover, by forming the control module with a suitable combination of resistive and reactive components, so-called shunts, selected frequencies can be brought to be damped with particular efficiency. Advantageously, such frequencies are the so-called “own-frequencies” of the exposed “own-modes” of the object, which are those preferably being excited.
Conversely, a piezo element may be compressed or elongated by an electric voltage being applied over the piezo element; during which the same may be used as a control or operating device (actuator). This can be used for active vibration control by selecting the polarity of the applied electric voltage in such a way that the mechanical stress of the operating device acts in the opposite direction, as an external, mechanical stress. The emergence of vibrations is suppressed by the fact that another kinetic energy, for instance rotation energy, is prevented from being converted to vibration energy. In doing so, the synchronization of the applied electric voltage relative to the external mechanical stress, the effect of which is to be counteracted, takes place by supplying a feedback signal from a deformation sensitive sensor to a control means in the form of a logical control circuit, e.g. a programmable micro-processor. The processor processes the signal to control the electric voltage applied over the operating device. The control function, i.e. the relation between the input signal from the sensor and the output voltage, may, in that connection, be made very complex. For instance, a self-learning system for adaptation to varying conditions is feasible. The sensor may consist of a separate, deformation sensitive device, e.g. a second piezo element, or be common with the operating device.
Examples of practical applications and present development areas for the utilization of piezo elements for vibration damping purposes, are described in Mechanical Engineering, November 1995, p. 76-81. Thus, skis for alpine skiing (K2 Four ski, K2 Corp., USA) have been equipped with piezo elements for the purpose of suppressing undesired vibrations, which otherwise decrease the contact with the ground, and thereby reduce the skier's prospects of a stable and controlled skiing. Furthermore, applications such as increased wing stability of aeroplanes, improved comfort in motor vehicles, suppression of vibrations in helicopter rotor blades and shafts, vibration control of process platforms for flexible manufacture, and increased accuracy of military weapons are mentioned. In information documents from Active Control eXperts (ACX) Inc., USA (a manufacturer of piezo elements), the vibration control of snowboards is also mentioned.
A method of the kind described in the introduction, as well as such a vibration damper and such a mechanical structure, respectively, is known from SE-A-9900441-8 (corresponding to U.S. application Ser. No. 09/913,271, the disclosure of which is hereby incorporated by reference herein).
This type of vibration damper is not suitable for force-induced vibrations. but only for regenerative, i.e. feed-back vibrations, which, e.g., arise in a tool during mechanical machining when a small disturbance gives a mechanical feed-back in the tool. Such a mechanical feed-back may cause an increasing oscillatory motion, and thereby an undesired uneven surface of the machined blank and reduced service life of the tool.
While SE-A-9900441-8 does not explicitly describe how to apply the damping force to the mechanical structure, the hitherto known way to dampen an oscillatory motion has been to generate a counter force in phase with the oscillatory motion. This procedure works well as long as low frequencies are concerned. At higher frequencies, i.e. above approximately 500 Hz, it is difficult to apply a counter force without phase errors. If a phase error arises, there is a risk of the oscillatory motion and the damping force ending up in an unbalanced state, and thereby partly amplifying each other, which in turn may lead to the oscillatory motion not being quenched to the desired degree. Thus, a presumption for such a vibration damping to work is that the counter-directed force is in phase with the oscillatory motion with a high degree of accuracy.
Other piezoelectric dampers are described in SE-A-9803605-6, SE-A-98003606-4, SE-A-9803607-2, U.S. Pat. No. 4,849,668, DE-A-199 25 193, EP-A-0 196 502, U.S. Pat. No. 5,485,053 and JP-A-63180401.
During chip removing machining, such as turning or drilling, it is not unrare for problems with vibrations to arise, particularly in cases in which the length of the shaft or tool outside the fastening device (a so-called overhang) is at least 3 times larger than the diameter thereof. One type of vibration is bending vibration, the shaft being curved to and fro and submitted to bending deformations. This phenomenon constitutes a common problem, for instance during turning, especially internal turning, where the shaft in the form of a boring bar has to be long in order to reach the area in the workpiece which is to be machined, at the same time as the diameter of the bar is limited by the dimension of the bore in which machining is carried out. During such drilling, turning and milling operations, where the distance to the workpiece is large, extenders are used, which frequently causes bending vibrations which not only lead to impaired dimensional accuracy and irregularities in the workpiece, but also to reduced service life of the tool and the cutting inserts or machining elements thereof.